o the
apparent size of flies. As this cause operates at different intervals of
time throughout a great portion of the earth's surface, it may be the
means of bearing not only plants but insects, land testacea and their
eggs, with many other species of animals, to points which they could
never otherwise have reached, and from which they may then begin to
propagate themselves again as from a new centre.

_Distribution of cryptogamous plants._--It has been found that a great
numerical proportion of the exceptions to the limitation of species to
certain quarters of the globe occur in the various tribes of cryptogamic
plants. Linnaeus observed that, as the germs of plants of this class,
such as mosses, fungi, and lichens, consist of an impalpable powder, the
particles of which are scarcely visible to the naked eye, there is no
difficulty to account for their being dispersed throughout the
atmosphere, and carried to every point of the globe, where there is a
station fitted for them.
Lichens in particular ascend to great elevations, sometimes growing two
thousand feet above the line of perpetual snow, at the utmost limits of
vegetation, and where the mean temperature is nearly at the freezing
point. This elevated position must contribute greatly to facilitate the
dispersion of those buoyant particles of which their fructification
consists.[853]

Some have inferred, from the springing up of mushrooms whenever
particular soils and decomposed organic matter are mixed together, that
the production of fungi is accidental, and not analogous to that of
perfect plants. But Fries, whose authority on these questions is
entitled to the highest respect, has shown the fallacy of this argument
in favor of the old doctrine of equivocal generation. "The sporules of
fungi," says this naturalist, "are so infinite, that in a single
individual of _Reticularia maxima_, I have counted above ten millions,
and so subtile as to be scarcely visible, often resembling thin smoke;
so light that they may be raised perhaps by evaporation into the
atmosphere, and dispersed in so many ways by the attraction of the sun,
by insects, wind, elasticity, adhesion, &c., that it is difficult to
conceive a place from which they may be excluded."[854]

The club-moss called _Lycopodium cernuum_ affords a striking example of
a cryptogamous plant universally distributed over all equinoctial
countries. It scarcely ever passes beyond the northern tropic, except in
one i